VLAN Trunk Protocol (VTP) reduces administration in a switched network.
When you configure a new VLAN on one VTP server, the VLAN is distributed
through all switches in the domain. This reduces the need to configure the same
VLAN everywhere. VTP is a Cisco-proprietary protocol that is available on most
of the Cisco Catalyst series products.

Note: This document does not cover VTP Version 3. VTP Version 3 differs
from VTP Version 1 (V1) and Version 2 (V2), and it is only available on
Catalyst OS (CatOS) 8.1(1) or later. VTP Version 3 incorporates many changes
from VTP V1 and V2. Make certain that you understand the differences between
VTP Version 3 and earlier versions before you alter your network configuration.
Refer to one of these sections of
VLAN
Trunking Protocol (VTP) for more information:

Note: This document does not cover VTP Version 3. VTP Version 3 differs
from VTP V1 and V2 and is only available on CatOS 8.1(1) or later. Refer to one
of these sections of
VLAN
Trunking Protocol (VTP) for more information:

VTP packets are sent in either Inter-Switch Link (ISL) frames or in
IEEE 802.1Q (dot1q) frames. These packets are sent to the destination MAC
address 01-00-0C-CC-CC-CC with a logical link control (LLC) code of Subnetwork
Access Protocol (SNAP) (AAAA) and a type of 2003 (in the SNAP header). This is
the format of a VTP packet that is encapsulated in ISL
frames:

Of course, you can have a VTP packet inside 802.1Q frames. In that
case, the ISL header and cyclic redundancy check (CRC) is replaced by dot1q
tagging.

Now consider the detail of a VTP packet. The format of the VTP header
can vary, based on the type of VTP message. But, all VTP packets contain these
fields in the header:

The configuration revision number is a 32-bit number that indicates the
level of revision for a VTP packet. Each VTP device tracks the VTP
configuration revision number that is assigned to it. Most of the VTP packets
contain the VTP configuration revision number of the sender.

This information is used in order to determine whether the received
information is more recent than the current version. Each time that you make a
VLAN change in a VTP device, the configuration revision is incremented by one.
In order to reset the configuration revision of a switch, change the VTP domain
name, and then change the name back to the original name.

By default, Catalyst switches issue summary advertisements in
five-minute increments. Summary advertisements inform adjacent Catalysts of the
current VTP domain name and the configuration revision number.

When the switch receives a summary advertisement packet, the switch
compares the VTP domain name to its own VTP domain name. If the name is
different, the switch simply ignores the packet. If the name is the same, the
switch then compares the configuration revision to its own revision. If its own
configuration revision is higher or equal, the packet is ignored. If it is
lower, an advertisement request is sent.

This list clarifies what the fields means in the summary advertisement
packet:

The Followers field indicates that this packet is followed by a
Subset Advertisement packet.

The Updater Identity is the IP address of the switch that is the last
to have incremented the configuration revision.

The Update Timestamp is the date and time of the last increment of
the configuration revision.

Message Digest 5 (MD5) carries the VTP password, if MD5 is configured
and used to authenticate the validation of a VTP
update.

When you add, delete, or change a VLAN in a Catalyst, the server
Catalyst where the changes are made increments the configuration revision and
issues a summary advertisement. One or several subset advertisements follow the
summary advertisement. A subset advertisement contains a list of VLAN
information. If there are several VLANs, more than one subset advertisement can
be required in order to advertise all the VLANs.

This formatted example shows that each VLAN information field contains
information for a different VLAN. It is ordered so that lowered-valued ISL VLAN
IDs occur first:

Most of the fields in this packet are easy to understand. These are two
clarifications:

Code—The format for this is 0x02 for subset
advertisement.

Sequence number—This is the sequence of the packet
in the stream of packets that follow a summary advertisement. The sequence
starts with 1.

The switch has received a VTP summary advertisement with a higher
configuration revision than its own.

Upon receipt of an advertisement request, a VTP device sends a summary
advertisement. One or more subset advertisements follow the summary
advertisement. This is an example:

Code—The format for this is 0x03 for an
advertisement request.

Start-Value—This is used in cases in which there are
several subset advertisements. If the first (n) subset
advertisement has been received and the subsequent one
(n+1) has not been received, the Catalyst only requests
advertisements from the (n+1)th
one.

Server—In VTP server mode, you can create, modify, and delete VLANs
and specify other configuration parameters, such as VTP version and VTP
pruning, for the entire VTP domain. VTP servers advertise their VLAN
configuration to other switches in the same VTP domain and synchronize their
VLAN configuration with other switches based on advertisements received over
trunk links. VTP server is the default mode.

Client—VTP clients behave the same way as VTP servers, but you cannot
create, change, or delete VLANs on a VTP client.

Transparent—VTP transparent switches do not participate in VTP. A VTP
transparent switch does not advertise its VLAN configuration and does not
synchronize its VLAN configuration based on received advertisements, but
transparent switches do forward VTP advertisements that they receive out their
trunk ports in VTP Version 2.

Off (configurable only in CatOS switches)—In the three described
modes, VTP advertisements are received and transmitted as soon as the switch
enters the management domain state. In the VTP off mode, switches behave the
same as in VTP transparent mode with the exception that VTP advertisements are
not forwarded.

VTP V2 is not much different than VTP V1. The major difference is that
VTP V2 introduces support for Token Ring VLANs. If you use Token Ring VLANs,
you must enable VTP V2. Otherwise, there is no reason to use VTP V2. Changing
the VTP version from 1 to 2 will not cause a switch to reload.

If you configure a password for VTP, you must configure the password on
all switches in the VTP domain. The password must be the same password on all
those switches. The VTP password that you configure is translated by algorithm
into a 16-byte word (MD5 value) that is carried in all summary-advertisement
VTP packets.

VTP ensures that all switches in the VTP domain are aware of all VLANs.
However, there are occasions when VTP can create unnecessary traffic. All
unknown unicasts and broadcasts in a VLAN are flooded over the entire VLAN. All
switches in the network receive all broadcasts, even in situations in which few
users are connected in that VLAN. VTP pruning is a feature that you use in
order to eliminate or prune this unnecessary
traffic.

Broadcast traffic in a switched network without
pruning

This figure shows a switched network without VTP pruning enabled. Port
1 on Switch A and Port 2 on Switch D are assigned to the Red VLAN. If a
broadcast is sent from the host connected to Switch A, Switch A floods the
broadcast and every switch in the network receives it, even though Switches C,
E, and F have no ports in the Red VLAN.

Broadcast traffic in a switched network with
pruning

This figure shows the same switched network with VTP pruning enabled.
The broadcast traffic from Switch A is not forwarded to Switches C, E, and F
because traffic for the Red VLAN has been pruned on the links shown (Port 5 on
Switch B and Port 4 on Switch D).

When VTP pruning is enabled on a VTP server, pruning is enabled for the
entire management domain. Making VLANs pruning-eligible or pruning-ineligible
affects pruning eligibility for those VLANs on that trunk only (not on all
switches in the VTP domain). VTP pruning takes effect several seconds after you
enable it. VTP pruning does not prune traffic from VLANs that are
pruning-ineligible. VLAN 1 and VLANs 1002 to 1005 are always
pruning-ineligible; traffic from these VLANs cannot be pruned. Extended-range
VLANs (VLAN IDs greater than 1005) are also pruning-ineligible.

By default, all switches are configured to be VTP servers. This
configuration is suitable for small-scale networks in which the size of the
VLAN information is small and the information is easily stored in all switches
(in NVRAM). In a large network, the network administrator must make a judgment
call at some point, when the NVRAM storage that is necessary is wasteful
because it is duplicated on every switch. At this point, the network
administrator must choose a few well-equipped switches and keep them as VTP
servers. Everything else that participates in VTP can be turned into a client.
The number of VTP servers should be chosen in order to provide the degree of
redundancy that is desired in the network.

Notes:

If a switch is configured as a VTP server without a VTP domain name,
you cannot configure a VLAN on the switch.

Note: It is applicable only for CatOS. You can configure VLAN(s) without
having the VTP domain name on the switch which runs on IOS.

If a new Catalyst is attached in the border of two VTP domains, the
new Catalyst keeps the domain name of the first switch that sends it a summary
advertisement. The only way to attach this switch to another VTP domain is to
manually set a different VTP domain name.

Dynamic Trunking Protocol (DTP) sends the VTP domain name in a DTP
packet. Therefore, if you have two ends of a link that belong to different VTP
domains, the trunk does not come up if you use DTP. In this special case, you
must configure the trunk mode as on or
nonegotiate, on both sides, in order to allow
the trunk to come up without DTP negotiation agreement.

If the domain has a single VTP server and it crashes, the best and
easiest way to restore the operation is to change any of the VTP clients in
that domain to a VTP server. The configuration revision is still the same in
the rest of the clients, even if the server crashes. Therefore, VTP works
properly in the domain.

There are some disadvantages to the use of VTP. You must balance the
ease of VTP administration against the inherent risk of a large STP domain and
the potential instability and risks of STP. The greatest risk is an STP loop
through the entire campus. When you use VTP, there are two things to which you
must pay close attention:

Remember the configuration revision and how to reset it each time
that you insert a new switch in your network so that you do not bring down the
entire network.

Avoid as much as possible to have a VLAN that spans the entire
network.